Justin Bieber wasn't serious about taking on Tom Cruise in a fight, or maybe he's simply changed his mind ... because he says Tom would kick his ass!
JB played it cool for the paps in L.A. Wednesday as he was leaving Hailey's fashion line office, telling them he was just goofing around when he randomly challenged the 56-year-old actor to an MMA fight and asked Dana White to make it happen.
He says he had just seen an interview with Cruise so he was on his mind, and decided to tweet out the bizarre challenge ... but it was all just a goof.
However, the Biebs says if it hypothetically were to happen, he'd have to get into shape ... because everyone knows Tom doesn't mess around.
And, he's got dad strength.
So now that that's been cleared up ... we just need to know the story behind his t-shirt.